High performance semiconductor manufacturing is largely a heuristic effort involving much trial and error. Thus, acquiring the know how to consistently achieve high semiconductor yields is largely labor intensive. A crucial part of this experience gathering is the timely receipt of accurate feedback on how minute semiconductor structures are formed on the semiconductor substrate. With timely and accurate feedback, the process engineer's optimization efforts can more effectively increase semiconductor yields. While all semiconductor manufacturers have their own recipe for success, all semiconductor manufacturers can benefit from the present invention's improvements.
Conventional instruments for measuring semiconductor structures include the following three types: (1) scanning electron microscopes, (2) atomic force microscopes, and (3) electrical test structures.
The scanning electron microscope (SEM) is an important instrument by which semiconductor manufacturers receive timely and somewhat accurate information of the semiconductor structures they are fabricating. The SEM provides means for observing and garnering some information on the physical dimensions of minute semiconductor structures.
The scanning electron microscope functions by directing a narrow beam of electrons at an observation target and measuring the electron beam's return signal with a detector. While the intricate details of how a SEM functions is beyond the scope necessary for the present discussion, the limitations of the SEM are illustrative of the need for the present invention.
Two major sources of error in the SEM output waveform include (1) secondary electron emissions and (2) "modulation" of the electron beam. The secondary electron emission problem arises in part from the excitation of the structure being irradiated with the electron beam and the structures surrounding the device under test (DUT). Secondary electron emissions, inter alia, may inject non-linear noise into the measurement system.
Modulation of the SEM electron beam may be a second factor in distorting the SEM measurement. Modulation of the electron beam may result from several different factors including electrical charge(s) accumulating in or on the DUT, the finite width (or limited resolution) of the scanning electron beam; limitations and non-linear response in and from the SEM detector; and limitations to the models used to extrapolate information from the SEM output waveform about the structure under inspection.
As device design rules continue to shrink the semiconductor structures, these limitations of the SEM for process inspections become more glaring.
As applied in practice, the limitations of the SEM make it difficult to accurately determine so called "critical dimensions" of the structure under inspection. A "critical dimension" may be defined as the physical dimensions of a semiconductor structure that fall within predetermined parameters. For example, FIG. 1 provides an illustration of a structure under inspection by SEM. The structure 12 has a width 14, a height 16 an angle denoting the ratio 18 of the width to the height of the structure (denoted by the term "alpha" or .alpha.). These three measurements may indicate one or more critical dimensions of the structure. The SEM, however, cannot readily detect the height and alpha of the semiconductor structure because of the distortion on the SEM output.
In FIG. 2, the electron beam 10 from the SEM is shown approaching the structure under inspection 12 from the left. FIG. 2 shows the secondary emission 20 and beam scattering effect induced into the beam from the device under test 22.
Additional distortion on the beam may be induced by electron absorption, e.g., electrical charging of the device under test. FIG. 3 shows an output waveform 30 of the SEM for the device under test and the structure under inspection. The waveform peaks 32 shown in FIG. 3 may give an indication of the relative distance of the edges 34 shown in FIG. 2 of the structure under test. The SEM output waveform may be input to a critical dimension-scanning electron microscope (CD-SEM) algorithm 40 to help determine an estimate of the critical dimension of the structure under test 42. This CD-SEM algorithm, shown in FIG. 3 as box 40, may approximate a critical dimension by determining the "distance" 42 between the algorithm-determined points on the SEM waveform 30.
The major short coming of this methodology is that the critical dimension is determined from the two dimensional width of the structure under test. Thus, the SEM does not use the height, slope and other three dimensional characteristics of the structure under test to determine critical dimensions.
The atomic force microscope (AFM) may also be used to measure semiconductor structures. The atomic force microscope measures the so called atomic force between the probe of the AFM and the structure under test. The so called atomic force is a force that acts between atoms when the atoms are in extremely close proximity. The AFM makes practical use of the atomic force phenomena by placing the AFM probe in very close proximity to the structure under test and slowly moving the probe across the structure under test. By using the atomic force to keep the AFM probe a predetermined (and extremely close) distance from the structure under test in conjunction with a means for precisely determining the probe location, the AFM may measure the physical dimensions of the structure under test much more accurately than the SEM. However, atomic force microscopy is a very slow and expensive process that limits its usefulness in providing timely and cost-effective feedback to the semiconductor manufacturer.